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United States Patent |
6,238,453
|
Rose
,   et al.
|
May 29, 2001
|
Producing stainless steels in parallel operated vessels
Abstract
A process for producing stainless steels, particularly special steels
containing chromium and chromium-nickel, in a smelting arrangement having
at least two vessels, for supplying a steel foundry. A charge having
mostly iron-containing raw scrap materials and partially carbon-containing
alloy carriers is melted in a first vessel. At a temperature of
1460.degree. C., the melt is decarburized by the injection of oxygen so as
to reduce the carbon content to less than 0.3%. The melt is heated to a
tapping temperature of between 1620.degree. C. to 1720.degree. C. and the
carbon content is subsequently reduced to 0.1%. A second charge is melted
in a second vessel simultaneously with the decarburizing of the first
charge in the first vessel.
Inventors:
|
Rose; Lutz (Duisburg, DE);
Vorwerk; Hartmut (Duisburg, DE);
Kappes; Horst (Duisburg, DE);
Ulrich; Klaus (Heiligenhaus, DE)
|
Assignee:
|
Mannesmann AG (Dusseldorf, DE)
|
Appl. No.:
|
117573 |
Filed:
|
July 31, 1998 |
PCT Filed:
|
January 27, 1997
|
PCT NO:
|
PCT/DE97/00171
|
371 Date:
|
July 31, 1998
|
102(e) Date:
|
July 31, 1998
|
PCT PUB.NO.:
|
WO97/28285 |
PCT PUB. Date:
|
August 7, 1997 |
Foreign Application Priority Data
| Jan 31, 1996[DE] | 196 05 020 |
| May 15, 1996[DE] | 196 21 143 |
Current U.S. Class: |
75/10.63; 75/10.48; 75/10.5; 75/10.51; 75/500; 75/531; 75/543; 75/551 |
Intern'l Class: |
C21B 013/12; C21C 007/072 |
Field of Search: |
75/10.5,10.51,10.48,10.63,10.64,525,551,500,531,537,540,543
266/900,216,225
|
References Cited
U.S. Patent Documents
3379815 | Apr., 1968 | Parker | 266/142.
|
3507642 | Apr., 1970 | Shaw | 75/551.
|
3575696 | Apr., 1971 | Rehmus | 75/382.
|
3746325 | Jul., 1973 | Freeberg et al. | 266/143.
|
4599107 | Jul., 1986 | Masterson | 75/552.
|
5264020 | Nov., 1993 | Ehle et al. | 266/142.
|
5520718 | May., 1996 | Keilman et al. | 75/508.
|
5547489 | Aug., 1996 | Inagaki et al. | 75/548.
|
5599375 | Feb., 1997 | Gitman | 75/10.
|
Foreign Patent Documents |
0035487 | Sep., 1981 | EP.
| |
0116720 | Aug., 1984 | EP.
| |
0331751 | Sep., 1989 | EP.
| |
754587 | Oct., 1957 | GB.
| |
Other References
Steel in the USSR, vol. 19, No. 2, Feb. 1989, London, GB, pp. 61-62,
XP000070901 Yu.V. Gravilenko: "Production of Stainless Steel".*
Database WPI Derwent Publications Ltd., London GB; AN 84-002177 c01
XP002035532 "Mfg Nitrogen-Containing Ultra Low Carbon Stainless Steel" &
JP 58197211 A (Nippon Stainless), Nov. 16, 1983.
|
Primary Examiner: King; Roy
Assistant Examiner: McGuthry-Banks; Tima
Attorney, Agent or Firm: Cohen, Pontani, Lieberman & Pavane
Claims
What is claimed is:
1. A process for smelting and refining stainless steel in a smelting
arrangement having electrodes and at least two vessels for supplying a
steel foundry, comprising the steps of:
a) melting using electrodes, in a first vessel, a first charge
substantially including at least one of solid and liquid metallic
iron-containing raw materials, and partially including carbon-containing
alloy carriers, so as to produce a melt covered with slag;
b) decarburizing the melt after reaching a temperature of 1460.degree. C.,
to a carbon content of less than 0.3% by injecting one of oxygen and
oxygen mixtures;
c) heating the melt to a tapping temperature of 1620.degree. C. to
1720.degree. C.;
d) subsequently bringing the melt to a final carbon content of less than
0.1%, steps a)-d) being carried out in the first vessel; and
e) melting a second charge in a second vessel during the decarburizing of
the first charge in the first vessel by pivoting the electrodes from the
first vessel to the second vessel, and repeating steps b) through d) in
the second vessel whereby all the process steps for smelting and refining
are carried out respectively in each of the vessels so that the vessels
function in a parallel, staggered fashion.
2. The process according to claim 1, further comprising reducing
substantially all melt slag by a reducing agent, and subsequently tapping
the reduced melt slag together with the metal, after the final carbon
content and tapping temperature are reached.
3. The process according to claim 2, wherein the reducing agent is one of
ferrosilicon, silicon and aluminum.
4. The process according to claim 1, wherein the decarburizing step
includes injecting one of oxygen and oxygen mixtures by top-blowing in
combination with at least one of bottom blowing and side blowing.
5. The process according to claim 1, wherein the decarburizing step
includes decarburizing the melt to a final carbon content of up to 0.05%,
for an oxygen injection period of 20 to 40 minutes.
6. The process according to claim 1, further comprising, terminating the
oxygen injecting at a carbon content of approximately 0.2% to 0.3% and a
temperature of approximately 1650.degree. C., reducing slag with one of
ferrosilicon and aluminum, emptying the metal and the slag together into a
ladle, removing the slag by decanting and by scraping off, and bringing
the metal in the ladle to a desired final carbon content of less than
0.1%.
7. The process according to claim 1, wherein step e) includes vacuum
degassing the melt to a desired final carbon content of less than 0.1%.
8. The process according to claim 1, wherein the melting steps include
melting the charge by means of electrical energy.
9. The process according to claim 2, further comprising, adding oxygen
during the melting step so as to cause oxidation of any amount of silicon
that may be present.
10. The process according to claim 9, wherein the oxygen is added through a
door lance.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a process for producing stainless
steels, and particularly for producing special steels containing chromium
and chromium-nickel, in a smelting arrangement having at least two vessels
for supplying a steel foundry.
2. Description of the Related Art
Usually, an electric furnace of conventional construction is used in the
production of chromium-containing, and chromium-nickel-containing
stainless steels. The electric furnace is constructed as a D.C. or A.C.
furnace in which scrap and other iron-containing metallic raw material,
e.g., pig iron or DRI (Direct Reduced Iron), are melted together with an
adequate amount of alloying elements or alloy carriers. The raw or base
material which is melted for this purpose is tapped off into a ladle at a
temperature of 1670.degree. C. to 1700.degree. C. The ladle is
subsequently emptied into a converter wherein the melt, which contains
approximately 2.5% carbon and approximately 1% silicon, is first oxidized
or refined by oxygen. The carbon content is next reduced by mixtures of
oxygen and nitrogen, and later by mixtures of oxygen and argon.
Depending on the application of different process techniques,
decarburization is carried out to produce a final carbon content of less
than 0.1%. Resulting chromium losses in the slag must then be recovered by
reduction with ferrosilicon or secondary aluminum.
Further, it is known in a three-step process technique to tap off the metal
from the converter at carbon contents of approximately 0.2% to 0.3% and
subsequently to bring the metal to the final carbon content in a separate
vacuum oxidation process.
A disadvantage that the previously known methods have in common is that
decanting or reladling the melt one or more times results in high
temperature losses. These losses must be compensated. For by using a high
tapping temperature resulting in a high amount of energy consumption in
the primary melting vessel, such as the electric arc furnace. In addition
to the high amount of energy consumption, the known methods
disadvantageously cause increased electrode and refractory wear in the
electric furnace. Furthermore installation of the converter required for
the second process step requires substantially high construction heights
for a surrounding building in order to accommodate a blowing lance and
exhaust gas system.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process for the
production of stainless steels having fewer process steps and lower energy
consumption in the individual process steps than in the known art. A
further object is to provide operating equipment that can be constructed
at a lower height.
The process of the present invention begins by melting, in a first vessel,
a first charge having mostly iron-containing raw scrap materials and
partially carbon-containing alloy carriers. At a temperature of
1460.degree. C., the melt is decarburized by the injection of oxygen, such
as by blowing, so as to reduce the carbon content to less than 0.3%. Next
the melt is heated to a tapping temperature of between 1620.degree. C. to
1720.degree. C. The carbon content is then reduced to 0.1%. The process of
melting a second charge in a second vessel is accomplished simultaneously
with the decarburizing of the first charge in the first vessel.
The process according to the present invention is carried out in a smelting
device having at least two vessels being operated in parallel. Either
electrodes for melting the charge, or blowing lances for top-blowing
and/or blowing in oxygen and oxygen mixtures can be used. The vessels thus
serve initially as smelting units and then as refining units. This has the
advantage that the melt can be processed and brought to a desired
temperature without experiencing temperature losses caused by decanting.
Scrap, ferronickel, nickel, ferrochromium and other metallic
iron-containing raw materials are melted in each of the vessels at
different times, preferably by electrical energy. This results in a base
metal containing mostly iron and having a chromium and nickel content
close to the final analysis of the steel quality to be produced,
particularly as austenites, ferrites and martensites.
In an advantageous embodiment of the present invention, when using high
carbon-containing and/or high silicon-containing ferrochromium, oxygen is
blown in by a lance so that the silicon content is reduced. After a melt
temperature of 1500.degree. C. to 1600.degree. C. is reached, in the same
vessel, the electrode arm is swiveled out. An oxygen lance is swiveled in,
which together with nozzles located in a bottom in a side wall of the
vessel, oxidizes the melt with oxygen. Of course, mixtures of oxygen and
nitrogen, argon, hydrocarbon, and steam can also be used to oxidize the
melt. For an average blowing period of 20 to 40 minutes, and at an oxygen
injection rate of 0.1 to 2.0 Nm.sup.3 /t.times.min., for the oxygen lance
and the injection nozzles, the melt is decarburized to a final carbon
content of 0.10% to 0.015%.
The amount of heat liberated by the blowing process can be utilized to add
coolant, as for example, Ni, FeNi, ferrochromium, scrap and other
iron-containing metallic raw materials such as pig iron masses, DRI or
alloying agents, in order to adjust the target analysis and target
temperature.
After blowing, the slag is reduced by reducing agents such as ferrosilicon,
aluminum or secondary aluminum with the addition of slag developers such
as lime and fluorspar for recovering oxidized chromium. The produced steel
and/or the slag are/is tapped off. The vessel is again filled with scrap
and alloy carriers, the electrodes are swiveled in, and the scrap and
alloy carriers are melted by the electrodes.
The smelting process can and the subsequent blowing process run
successively in each of the respective vessels of synchronously between
the vessels. After 80 to 120 minutes, a melt can be prepared from one
vessel, or in the case of synchronous production of both vessels, a melt
is prepared every 40 to 60 minutes for further-processing, in a continuous
casting plant.
The simultaneous use of two vessels not only has the advantage, of
continuously supplying a continuously casting plant, but is also
advantageous with respect to energy. After the smelting in the first
vessel, for example, the-still hot-electrodes drawn out of the first
vessel can then be introduced into the second vessel to begin the smelting
therein. This process reduces energy consumption and electrode loss.
The blowing process is carried out to the lowest carbon content which
naturally leads to high stress of the refractory material in the vessel
hearth or bottom. Therefore, the blowing process, in an advantageous
embodiment of the present invention, is terminated when a carbon content
of 0.2% to 0.4% is reached. In this embodiment, metal and slag are emptied
together into a ladle. The slag is removed by decanting and by scraping.
The ladle with the metal melt is then transferred into a vacuum vessel, as
is known, so that by blowing oxygen the metal melt is refined to a final
carbon content.
Utilizing this process, the typically elaborate installation of a converter
for the blowing process is not necessary particularly in the preferred
embodiments of the present invention, so that investment costs for the
process can be decisively reduced. Furthermore, there is no
energy-consuming decanting of the base metal which has been melted by
electrical energy, especially from the transporting ladle into the
converter, in order to refine the carbon content by using oxygen.
For a particularly high degree of oxidation of silicon, another preferred
embodiment of the present invention adds that oxygen during the smelting
of the charge. A door lance is used in this embodiment so as to avoid
special construction steps.
An example of the invention is shown in the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a diagram of the individual process steps.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
First, considering process phase 1, when furnace vessel 1 is at step A,
there is a small liquid pool 50 in the furnace vessel 1 on which a newly
poured charge 60 is located. The furnace vessel 1 is closed by means of a
cover 30 through which electrodes 40 project into the upper vessel 20 of
the furnace 1. In step B, the charge 60 is melted by electrical energy
provided by the electrodes 40. In so doing, the liquid bath level 50 rises
in the lower vessel 10 of the furnace 1. In step C, the charge 60
comprising essentially scrap, ferronickel, nickel, ferrochromium and other
iron-containing metallic materials, is melted virtually completely to
liquid base metal 50. When using high carbon-containing and/or high
silicon-containing alloying means, oxygen can be blown in by means of a
first lance 70, so that the silicon content is reduced.
In step D, the base material is completely melted, and the melt has a
temperature of 1500.degree. C. to 1600.degree. C. In vessel 1, the
electrode device 40 is removed from the vessel 1 and a second oxygen lance
80 is swiveled in for changing to process phase 2. In the present diagram,
process phase 2 is shown in furnace vessel 2 because this process phase
runs in vessel 2 at the same time as process phase 1 runs in furnace
vessel 1 (and vice versa).
Now turning to the process phase 2, in step A, the melt is oxidized with a
lance 100 and with bottom nozzles 90 or side nozzles, wherein oxygen or an
oxygen mixture is used. The bottom nozzles 90 or side nozzles 90 are
concentric nozzles 90, having an outer tube, an annular clearance, and a
central tube. Oxygen, or an oxygen mixture comprising O.sub.2 +N.sub.2,
O.sub.2 +Ar, or O.sub.2 +air, is introduced through the central tube.
N.sub.2 or Ar or hydrocarbon or steam or a mixture of these gasses is
blown in through the annular clearance at the same time.
In step B, after the final carbon content is reached at a tapping
temperature of 1620.degree. C. to 1680.degree. C., the melt slag is
reduced by a reducing agent such as ferrosilicon or aluminum in order to
recover oxidized chromium. The metal and slag are then tapped together.
In step C, the melt can be decarburized to a final carbon content of 0.05%,
as shown in alternative 1, or, as shown in alternative 2, transferred
after separation of the slag to a vacuum installation at a carbon content
of 0.2 to 0.4% and brought to the desired final carbon content therein. A
finished steel is then tapped.
In step D, furnace 2 is filled with a new charge of scrap and partially
with alloy carriers containing carbon, wherein a liquid pool can be
located in the furnace vessel 2.
As was indicated above, the process is then repeated, beginning with Step
A.
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